Amniocentesis is performed in pregnant women to determine genetic abnormalities prior to birth. It is used on women over 35 years of age, or with a family history of genetic disorders, chromosomal defects, mental retardation or RH incompatibility. Generally it is performed by placing a long 22 gauge needle into the uterus and withdrawing about 20 ml of amniotic fluid to be sent to the lab for analysis. This is usually performed between the 14th and 24th week of gestation. In addition to obtaining genetic information, the amniotic fluid also can be tested biochemically for Neural Tub Defects (alpha-fetoprotein), RH factor complications, creatine concentration, absence of bilirubin among others. The major problem with amniocentesis is that it can be performed only after the 14th week, and preferably not prior to the 16th week, and it can take from ten days to four weeks to cultivate a readable specimen. An abnormality such as Down's Syndrome (trisomy 21) may be the outcome of this test and most patients have an awesome dilemma whether to repeat the test (remote chance of error) or abort the fetus at that time. Repeating the test may result inconfirmation coming too late to safely abort the fetus. The only other technique for determining genetic information is called chorionic villus sampling (CVS) and can be done in the 8-10th week of gestation. DVS is still experimental and currently has a higher spontaneous abortion rate than ultrasound guided amniocentesis.